The present invention relates generally to burner assemblies, and more particularly, is directed to a register and/or gas pilot for a burner assembly.
Burner assemblies in which gas, oil, coal and/or other combustible materials are mixed with air are well known in the art. Such burner assemblies are generally associated with industrial boilers and furnaces, and comprise a structure known as a register which is usually mounted at the base of the furnace or boiler. The register contains appropriate fuel and air inlets, and houses the burner gun that serves to ignite the fuel. Thus, the combustible material, such as oil, coal and/or gas, enters the register through appropriate inlets formed in the register. In order to provide efficient mixing of the air with the fuel, a plurality of entry ports are generally positioned within the annular side wall of the register, so that the air impacts the combustible material at an angle thereto in order to provide enhanced mixing. Additionally, the entry ports within the annular side wall may be inclined so as to provide a tangential spin to the air supplied to the register.
With such conventional arrangements, the problem of adequate mixing of the air with the combustible material still remains. In other words, it is still desirable to obtain more efficient and effective mixing of the air and fuel. In this regard, the particulate combustible material, such as the atomized oil and pulverized coal, is not fully burned.
Related to this problem of inadequate air balance, that is, inadequate mixing of the fuel and air, there is the problem of reducing emissions, such as carbon monoxide (CO), hydrocarbons (HC), and nitrous oxides (hereinafter referred to as NOx) resulting from oxidation of nitrogen in the air. It is therefore desirable to provide a desired temperature in the burner in order to reduce these emissions, for example, to crack the nitrogen and thereby reduce NOx. With conventional arrangements, however, in order to reduce emissions, some of the output flue gas is recirculated with the input air for combustion. This, however, lowers the temperature in the register, and provides a lower oxygen content for burning, thereby decreasing the efficiency of the burner. Even when preheated air is used, full combustion efficiency is not realized.
There are various reasons, in addition to the above, why conventional register/burner assemblies do not fully reduce the particulate fuel to a gaseous state and do not fully reduce emissions. First, the plane of fuel emission is downstream of the entrance of the combustion air. Thus, when the combustion air meets with the fuel, it competes as a heat receiver with the particulate fuel so that complete burning of the particulate to the point of a state change and vaporization, is delayed. Secondly, because the plane of fuel emissions is downstream of the combustion air, much more air, for example, an order of magnitude more air, meets with the fuel than is required for an immediate sub-stoichiometric (shortage of air) combustion stage. Third, with the plane of fuel emissions downstream of the combustion air, the flame mass immediately radiates to the cooler walls of the furnace prematurely terminating the nascent combustion stage. In the case of a predominantly water wall furnace, the flame is actually severely chilled prior to completion of the sub-stoichiometric or nascent stage of combustion. Fourth, divergent flames hasten the chilling of the flame mass due to the flames's close proximity to the water wall.
Fifth, in the case of inside-mix atomizing steam guns for liquid fuels, considerable steam is condensed when meeting the oil within the gun's mixing chamber prior to nozzle emission. The condensate is harmful to the substoichiometric stage combustion process since it serves as an additional heat sink at a critical time and becomes oil coated particulate. When, with its heavier mass, the oil coated particulate approaches the adjacent water wall, the oil coating becomes chilled to soot.
As discussed in U.S. Pat. No. 4,297,093, various methods have been used for suppressing the generation of NOx and other emissions, such as reduction of the flame temperature, reduction of oxygen concentration in the combustion zone and shortening of the stay time of the combustion gas in the combustion zone of high temperature. However, as described therein, the adoption of these techniques also poses various problems concerning stability of the flame, emission of unburnt substances and smoke, the responsive characteristic to the fluctuation of load, thermal efficiency, the cost of modification of the boiler, increase of the fuel consumption, and the like. U.S. Pat. No. 4,297,093 thereby discloses an arrangement for reducing NOx by utilizing a swirler to provide a small scale of turbulence to the combustion air. The swirler is located in the vicinity of the fuel injection port. The use of a swirler, however, reduces the efficiency of the burner and adds another element thereto.
Related to the turbulence discussed above, it is known that the flow velocity of a combustible mixture is reduced when an obstacle is placed in the flow path thereof. Accordingly, the chances for the flame speed to match the flow velocity at some region in the flow field, a requirement of flame stabilization, are improved. If the obstacle is a bluff body, that is, a non-streamlined body, as the fluid is accelerated, a flow velocity is reached where the adverse pressure gradient downstream from the obstacle is strong enough to set up a recirculating vortex system in the wake of the bluff body, as taught by Combustion Aerodynamics, J. M. Beer and N. A. Chigier, Halsted Press Division, John Wiley and Sons, Inc., New York, pages 68 and 73.
In order to solve the problems associated with conventional registers, it has been proposed in U.S. Pat. No. 4,629,416, having a common assignee herewith, to use a bluff body register. Such a bluff body register provides an excellent air balance, regardless of the fuel utilized. Further, such a bluff body register provides maximum turbulence for air entering the register with a pressure drop, resulting in enhanced mixing of the air and fuel, while substantially reducing the NOx.
Specifically, such a bluff body register includes an annular wall, with a plurality of bluff body elements circumferentially spaced about the annular wall in a plurality of axially spaced rows for supplying air to the register. A combustible material is supplied to the register, and a bluff body disc is positioned within each bluff body element for enhancing mixing of the combustible material and the air within the register. Thus, as the air enters the chamber of the register through each bluff body element, there is a resultant pressure drop, whereby the air is caused to disperse through the chamber and thereby mix with the gas or oil fuel. Further, the bluff body discs create toroidal eddies that increase the turbulence of the air entering the register by increasing the velocity and pressure drop thereof, so as to provide enhanced mixing of the fuel and air, and thereby a more efficient burner assembly. As a result, there is a reduction of NOx, without the necessity of providing a swirler at the air input. There is also no need to provide a recirculation of flue gases. This is because a greater amount of burning occurs in the register, which results in a desired temperature and thereby results in cracking of the fuel-bound nitrogen in the sub-stoichiometric zone in the register.
With such a bluff body register, the plane of all fuel emissions is upstream of the combustion air. Further, the combustion air is admitted in stages. Thus, the particulate fuel emissions receive combustion air in a series of stages as ideally required, and without chilling. Since all of the sub-stoichiometric combustion takes place within the confines of the air-cooled register, the flame leaving the register and entering the furnace is totally gaseous, that is, there is full conversion to the gaseous state of all free carbon, all forms of hydrocarbons, all free sulfur, sulfur compounds and any other combustibles in the fuel. Further, with sub-stoichiometric combustion being completed in stages without chilling, no hard soot is formed. The low micron size ash is then "dry" and is free to pass through the furnace, convention banks and air heater or economizer without sticking to any surfaces.
Further, the bluff body elements admit combustion air radially and generate extremely high turbulence, without any rotational spin of the flame envelope, thus insuring that the flame leaving the register is non-divergent or coherent. Soft soot is therefore avoided or minimized, with the flame ends not being in close proximity to the water walls.
Still further, the bluff body register of this patent also serves to prevent any surface migration of carbon monoxide or hydrocarbons. More importantly, the solution to the completion of sub-stoichiometric combustion within the confines of the register is also the solution to reducing NOx formation due to fuel-bound nitrogen, and additionally, a reduction of thermally formed NOx. The elimination or substantial reduction of NOx due to fuel-bound nitrogen, is due to sufficient heat generated by the sub-stoichiometric combustion, in combination with a long sub-stoichiometric flame retention time within the register. Thermally formed NOx is minimized due to the extended sub-stoichiometric region and the non-divergent flame with low penetration of the hotter flame core by the combustion air from the last stage of bluff body elements. Thus, the air from the last row of bluff body elements serves the role of excess post-combustion air.
However, it has been found from experience that the construction of such a register is time consuming and costly. Specifically, a plurality of holes must be cut by a flame torch in the annular wall. In large burner assemblies, this could result in hundreds of holes being cut. Then, the holes must be honed, which is a laborious task. Thereafter, each bluff body element is seated within a hole and welded thereto.
In addition to the aforementioned problems with respect to registers, there are additional problems with conventional gas pilots for use therewith. Specifically, because of the structural arrangement of conventional gas pilots, a gas rich mixture is used. This, however, tends to foul the spark plug therein, resulting in more frequent replacement of the spark plug and more down time for the gas pilot.